cmd/internal/pkgpath: use MustHaveExec in TestToSymbolFunc

ToSymbolFunc executes a command using exec.Command.

Change-Id: Ic6c274bdc23050d021ec98ba67748338ac34f12c
Reviewed-on: https://go-review.googlesource.com/c/go/+/435236
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

src/cmd/internal/pkgpath/pkgpath_test.go

@@ -5,6 +5,7 @@
 package pkgpath
 
 import (
+	"internal/testenv"
 	"os"
 	"testing"
 )
@@ -34,6 +35,8 @@ func init() {
 }
 
 func TestToSymbolFunc(t *testing.T) {
+	testenv.MustHaveExec(t)
+
 	const input = "pä世🜃"
 	tests := []struct {
 		env     string